I tried replacing / by \, but this is very tricky.

Why tricky? You just need to do a

$dirname =~ tr(/)(\\);
[download]

I think this test is important, because at the moment, we still *guess* that UNC pathes with forward slashes are the problem, but we don't know for sure.

You could just write a tiny example program and try out various styles for specifying a path to Tk functions...